The present invention relates to a backlighting device for use with display panels that illuminates transmissive or semi-transmissive panels from the rear side.
Liquid-crystal displays provided with a backlighting mechanism that is thin and which allows for easy viewing of information on the screen are used with recent models of laptop or book type word processors or computers. The backlighting mechanism in common use adopts an "edge lighting" method in which a linear light source such as a fluorescent tube is provided in proximity to one end portion of a transmissive light conducting plate as shown in FIG. 1. A most common type of devices that operate on this edge lighting method is shown in FIG. 2; a plurality of light diffusing elements are formed in dots or stripes on one face of a light conducting plate, which is almost entirely covered with a specular reflecting plate or a light diffusing and reflecting plate whereas the opposite face of the light conducting plate (from which light exits) is covered with a light diffusing sheet.
In addition, as is often the case today, backlighting devices are driven with a battery and a further improvement in the efficiency of conversion from power consumption to luminance is desired. To meet this need, it has been proposed that a sheet made of a light-transmissive material that has a multiple of prisms or raised structures having ridgelines at small intervals on the same side in such a manner that said ridgelines are substantially parallel to one another should be provided on the light emitting surface of the backlighting device, whereby the light it emits is provided with sufficient directivity to increase the brightness in a direction normal to the exit face.
However, the sheet itself is poor in lighting diffusing performance, so it does not have sufficient ability to hide the light diffusing elements formed on the light conducting plate and this has caused a problem in that the shape of the light diffusing elements is seen through the sheet. If the shape of the light diffusing elements is seen through the sheet, uniform areal light emission cannot be achieved.
With a view to solving this problem, it has been proposed that the sheet be provided with light diffusing quality by various methods such as coating the transmissive sheet itself with a light diffusing substance or forming a randomly rough surface on the sheet. However, these approaches have caused another problem in that the sheet's ability to impart directivity to light deteriorates to lower the brightness of the exit face.
It has also been proposed that and a separate light diffusing sheet be used in superposition; however, this arrangement increases the thickness of the backlighting device by the amount that corresponds to the thickness of the light diffusing sheet, thereby making it impossible to satisfy the requirement for fabricating a thinner backlighting device. Further, the approach under consideration is not necessarily satisfactory from the viewpoint of the brightness of the exit face.
If the distance between adjacent light diffusing elements to be formed on the light conducting plate were reduced to a very small value, say, 50 .mu.m or less, the individual light diffusing elements would become practically indiscernible but, as a matter of fact, it is technically difficult to form the light diffusing elements at such small spacings.
FIG. 3 illustrates a method for partially covering the light conducting plate with a light scattering and transmissive substance and/or a light diffusing and reflective substance in such a way as to provide a uniform luminance distribution throughout the light emitting surface of the backlighting device; the partial covering may be in dots as shown in FIG. 3 or in stripes as shown in FIG. 10 in such a way that the coverage per unit area of the light conducting plate increases progressively with the increasing distance from the light source.
The demand for reducing the thickness of laptop or booktype word processors and personal computers is ever growing today and one of the topics under current review by manufacturers is to adopt even thinner light conducting plates in the backlighting mechanism. However, if one wants to have a uniform luminance distribution throughout the light emissive surface of a very thin light conducting plate (particularly 2 mm or less), the coverage with a light scattering and transmissive substance and/or a light diffusing and reflective substance per unit area of the portion of the light conducting plate which is near the light source must be reduced (otherwise, the brightness of the portion close to the light source will become much higher the in the other portions, thus leading to a failure in providing a uniform luminance distribution throughout the emissive surface).
Consider, for example, the case where TiO.sub.2 as a light scattering and transmissive substance and/or a light diffusing and reflective substance is applied in a dot pattern on the surface of a Poly methyl methacrylate (PMMA) light conducting plate (250 mm.times.150 mm), with the individual dots being formed on the points of intersection of imaginary lines on the light conducting plate that are spaced on a pitch of 1 mm. If the thickness of the light conducting plate is 3 mm, a dot diameter of 420 .mu.m (14% coverage) for the portion of the light conducting plate which is close to the light source is sufficient to provide a uniform luminance distribution throughout the emissive surface but if the thickness of the light conducting plate is halved to 1.5 mm, no uniform luminance distribution is insured throughout the emissive surface unless the dot diameter is reduced to 220 .mu.m (3.8% coverage).
If the dot diameter is reduced, the dot-to-dot spacing is increased (in the case of stripes, the distance between adjacent stripes is increased) and the conventionally used light diffusing sheet is unable to diffuse the incoming light by a sufficient degree to prevent the shape of the individual dots from being seen through the sheet.
If, with a view to solving this problem, one uses a plurality of light diffusing sheets (in the case just described above, a single polycarbonate light diffusing sheet 0.2 mm thick suffices if the thickness of the light conducting plate is 3 mm but if the thickness of the light conducting plate is reduced to 1.5 mm, the shape of the individual dots is visible unless three such sheets are used in superposition), the thickness of the backlighting device will increase accordingly but this is not preferred for the purpose of fabricating thinner backlighting devices. In addition, the number of device components will increase to result in a higher cost.
Another problem associated with the decreasing dot diameter is that it becomes technically difficult to form dots on the surface of the light conducting plate (the same problem arises with stripes). The light scattering and transmissive areas and/or the light diffusing and reflective areas are conventionally formed on the surface of the light conducting plate by printing techniques such as screen printing but the printing technology has an inherent limitation in that the printing yield drops if the dot diameter decreases to levels near 200 .mu.m. If the printing yield decreases, the cost of the light conducting plate produced becomes high.